A continuously variable transmission (referred to hereafter as CVT) for a vehicle using a V-belt is for example disclosed in Tokkai Sho 61-105347 published by the Japanese patent office in 1986.
This CVT comprises a drive pulley and driven pulley around which the belt is looped. The widths of the grooves of the pulleys are varied according to the oil pressure, and the radius of the contact part of the V-belt and the pulley varies according to the groove width.
An oil pressure (line pressure) corresponding to the input torque to the drive pulley acts on the driven pulley, and an oil pressure (primary pressure) which is the line pressure adjusted by a control valve acts on the drive pulley. The contact friction between the belt and the pulleys is basically maintained by the line pressure.
In CVT speed change control, the controller computes a target speed change ratio based on the accelerator depression by the driver and the vehicle speed at that time, and the primary pressure is made to vary relative to the line pressure so that the real speed change ratio coincides with a target speed change ratio.
However, as the line pressure is set to a value depending on the torque as mentioned above, the line pressure decreases when the driver releases the accelerator pedal corresponding to the decrease of input torque, and the contact friction also decreases.
At this time, when the contact friction decreases, the contact radius between the driven pulley and the V-belt decreases while the contact radius between the drive pulley and the V-belt increases due to the centrifugal force which acts on the V-belt. As a result, the speed change ratio decreases, i.e. the speed tends to increase.
In the automatic speed change mode wherein the speed change ratio frequently varies according to the driving conditions, the driver does not experience discomfort even when a speed change occurs due to release of the accelerator pedal. However in the fixed speed change mode where the speed change ratio is fixed at a certain value by the driver, the driver does experience discomfort if the speed change ratio suddenly changes when the accelerator pedal is released.